Amino acid 'X' contains phenolic hydroxy group and amino acid 'Y' contains amide group. 'X' and 'Y' respectively are
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Remember the side chains (R-groups) of the common amino acids.
- $\text{Tyrosine}$ has a phenolic group (and is classified as an aromatic amino acid).
- $\text{Serine}$ and $\text{Threonine}$ have aliphatic alcohol groups.
- $\text{Asparagine}$ ($\text{Asn}$) and $\text{Glutamine}$ ($\text{Gln}$) have amide groups.
Step 1: Identify the amino acid X with a phenolic hydroxy group.
A phenolic hydroxy group is a hydroxyl ($\text{-OH}$) group attached directly to a benzene ring.
We check the standard amino acids (AAs):
- $\text{Serine}$ ($\text{Ser}$) has an aliphatic primary alcohol ($\text{-CH}_2\text{OH}$).
- $\text{Tyrosine}$ ($\text{Tyr}$) has a $p$-hydroxyphenyl side chain ($\text{-CH}_2\text{-Ph-OH}$), which contains a phenolic hydroxy group.
Therefore, amino acid X is $\text{Tyrosine}$.
Step 2: Identify the amino acid Y with an amide group.
An amide group has the structure $\text{-CONH}_2$.
We check the standard amino acids:
- $\text{Asparagine}$ ($\text{Asn}$) has a side chain with an amide group ($\text{-CH}_2\text{CONH}_2$).
- $\text{Glutamine}$ ($\text{Gln}$) has a side chain with an amide group ($\text{-CH}_2\text{CH}_2\text{CONH}_2$).
Since the options list $\text{Tyr}$ (X) with either $\text{Gln}$ or $\text{Asn}$, and $\text{Gln}$ is the second member of option (D), let's check the options.
Option (D) lists $\text{Tyr}$ ($\text{X}$) and $\text{Gln}$ ($\text{Y}$). $\text{Gln}$ contains an amide group.
Option (C) lists $\text{Thr}$ ($\text{X}$, which is wrong) and $\text{Asn}$ ($\text{Y}$, which is correct).
Step 3: Conclude the final correct option.
Amino acid X ($\text{Tyr}$) has a phenolic group.
Amino acid Y ($\text{Gln}$) has an amide group.
The pair (X, Y) is $(\text{Tyr}, \text{Gln})$.
